1. Field of the Invention
The present invention relates to pressurized nuclear reactors, (PWRs), particularly to nuclear reactor components, and more particularly to head-mounted, removable incore instrumentation, such as thimble sheath, mandrel, and lead wires as used in advanced light water reactors such as the Westinghouse AP600.
2. Brief Description of the Prior Art
Current projections indicate that electrical energy demands will continue to grow, so that by the year 2010, there will be a need for approximately 117,000 to 322,000 MWe of new generating capacity in the United States. Although the growth in electricity demand continues to be strong, orders for new nuclear power plants have not kept pace, in part as a result of licensing delays, prohibitive construction costs, and public concerns relating to the safety of nuclear power. Nonetheless, increased awareness of the environmental and economic security risks attendant an excessive dependence on fossil fuels has led to a growing realization that nuclear power must play a major role in the world's energy future.
Of key importance in maintaining nuclear power as an attractive energy option for the future is the need to improve upon the safety, simplicity, reliability, and cost of construction and operation of nuclear reactors. This goal is being sought by the development of a new generation nuclear reactor, the Westinghouse AP600, which is part of a greatly simplified nuclear power plant, in terms of number of systems and equipment, operation inspections, maintenance, and quality assurance requirements relative to prior nuclear reactors.
As an example of simplified safety systems, the AP600 has replaced many of the active, powered safety systems of prior reactors with systems known as "passive," because they rely on natural forces such as gravity, convection, and evaporation, rather than operator action.
Part of the simplification of the AP600 reactor is the development of an integrated head package (IHP) that combines several separate components in one structure to simplify refueling the reactor. The goal of the integrated head package is to reduce outage time and personnel radiation exposure by combining operations associated with this movement into a single structure. This integrated head package connects to the reactor vessel head, controlled rod drive mechanism, seismic supports, cables, messenger tray, and cable bridge. Furthermore, the incore instrumentation system (IIS) is mounted above the reactor head and integral with the integrated head package, and is, therefore, moved with the integrated head package. The incore instrumentation system includes core thermocouples which are generally housed in instrument columns sealed using conoseals. These conoseals have caused increased outage durations due to leakage during plant start-up. The AP600 has replaced these conoseals with smaller, simpler tube fittings, (SwageLock.TM.) which are used in plants with bottom-mounted instrumentation.
When the reactor is operating, measurements of neutron flux must be carried out inside the reactor core in various places distributed along its cross section and along its height. These measurements are carried out by the incore nuclear instrumentation, which comprises the subject matter of the present invention. Previously, incore instrumentation was mounted on the bottom of the reactor vessel. Currently, however, there has been an emergence of DOE, EPRI, and NRC preference for incore instrumentation systems which do not penetrate the reactor vessel below the core, and which employ fixed incore detectors. Accordingly, incore instrumentation systems, such as fixed incore detection thimbles, are now being mounted on the reactor head, and inserted through the closure head and into the core of the PWR, as disclosed in U.S. Pat. No. 5,078,957, incorporated by reference herein.
A serious drawback to mounting the incore instrumentation system above the reactor head and integral with the integrated head package, however, is that this instrumentation, which previously was mounted at the bottom of the reactor vessel, may cause high levels of radiation exposure to workers who venture too close to the head-mounted instrumentation, especially during refueling, when the reactor head becomes an area of high activity.
The transport of the reactor vessel head containing 16 to 38 thimbles which have been irradiated in the core for 11/2 years requires the use of special procedures and possibly special shielding, especially when the head is lifted above the floor and appreciable gamma shine and scatter are present in containment. Removing all personnel from containment or locating them behind shadow shielding is one means of controlling occupational radiation exposure. The addition of a removable bottom steel shield plate 4 to 6 inches (7.6-10.2 cm) thick to the underside of the head 18 may also reduce occupational radiation exposure during transport to acceptable levels.
The major concern relative to the incore instrumentation system from a radiological perspective is the potential for over-exposure of personnel from the highly irradiated incore instrumentation system thimbles. Contact dose rates on the order of 10,000 R/hr are expected on the irradiated portion of the thimbles. Thus, it is mandatory that an unshielded line-of-sight to the irradiated material be avoided when the thimbles are in the withdrawn position. For example, the large diameter ventilation ducting at the top of the integrated head package in the current configuration of the AP600 severely compromises the integrity of the integrated head package shielding around the irradiated thimbles. These duct penetrations must be shielded prior to thimble withdrawal or an alternative ventilation design which does not afford a direct line-of sight through no shielding or minimal shielding must be incorporated.
Accordingly, it is an object of the invention to reduce the radiation exposure potential to personnel working near reactor head-mounted instrumentation.
It is another object of the invention to reduce radiation exposure resulting from irradiated head-mounted Incore Instrumentation thimbles.
These and other objects and advantages of the present invention will become more readily apparent as the following detailed description of the preferred embodiments proceeds.